A common calling pattern in mobile radio systems is a wide area group call in which multiple mobile users served by multiple sites can all converse. These calls may be implemented in trunked, simulcast and voted conventional networks. Base stations provide an air interface for the users. When a user speaks they are heard by all other group members on all sites participating in the group.
Sometimes two users at different sites speak at the same time. In analogue conventional and simulcast networks a centralised voter is commonly used to arbitrate or choose one signal to be broadcast by the base stations. Trunked networks often use a centralised conference bridge to mix the audio received from all of the participating sites. Circuit switched telephony also generally uses a centralised conference bridge. Packet voice systems typically use a range of approaches for conference calls including a centralised multi point controller.
Conventional systems have various drawbacks. There is a reduced choice of speech paths and calls must be sent through a voter, bridge, controller or other centralised component, even if the base stations serving the users are close together. This component can be a common point of failure. These systems are not fully scalable and the central component usually accommodates only a fixed number of base stations. It may be unnecessarily expensive in both small and large networks, and is simply another component in the overall structure of a network.